Method of data acquisition for three-dimensional imaging

ABSTRACT

Described are embodiments of methods of obtaining three-dimensional (3D) surface data of an object scene such as an intra-oral cavity. For example, measured surfaces may include the enamel surface of teeth, the dentin substructure of teeth, gum tissue and various dental structures. The methods can also be applied in medical applications and other applications in which 3D measurement data are acquired with maneuverable 3D measurement devices. In certain embodiments, the measurement device is positioned and translated to enable 3D data to be acquired for a backbone 3D data set. Subsequent controlled motion of the measurement device enables additional 3D data to be acquired and accurately joined to the backbone 3D data set.

RELATED APPLICATIONS

This application is a continuation application of co-pending U.S. application Ser. No. 13/217,595, filed Aug. 25, 2011 and titled “Method of Data Acquisition for Three-Dimensional Imaging,” which claims priority to and the benefit of the earlier filing date of U.S. Provisional Application No. 61/381,731, filed Sep. 10, 2010 and titled “Method of Data Processing and Display for a Three-Dimensional Intra-Oral Scanner,” the entireties of which applications are incorporated by reference herein.

FIELD OF THE INVENTION

The invention relates generally to three-dimensional (3D) imaging of the intra-oral cavity. More particularly, the invention relates to a method of acquiring 3D image data of an object scene using a plurality of 3D measurement scans and generating a complete 3D image from the scans.

BACKGROUND

In a typical dental or medical 3D camera or scanner imaging system, a series of two-dimensional (2D) intensity images of one or more object surfaces in an object scene is acquired where the illumination for each image may vary. In some systems, structured light patterns are projected onto the surface and detected in each 2D intensity image. For example, the projected light pattern can be generated by projecting a pair of coherent optical beams onto the object surface and the resulting fringe pattern varied between successive 2D images. Alternatively, the projected light pattern may be a series of projected parallel lines generated using an intensity mask and the projected pattern shifted in position between successive 2D images. In still other types of 3D imaging systems, techniques such as confocal imaging are employed.

In a dynamic 3D imaging system, a series of 3D data sets is acquired while the camera or scanner is in motion relative to the object scene. For example, the imaging system can be a wand or other handheld device that a user manually positions relative to the object scene. In some applications, multiple objects surfaces are measured by moving the device relative to the objects so that surfaces obscured from view of the device in one position are observable by the device in another position. For example, in dental applications the presence of teeth or other dental features in a static view can obscure the view of other teeth. A processing unit registers the overlapped region of all acquired 3D data to obtain a full 3D data set representation of all surfaces observed during the measurement procedure.

SUMMARY

In one aspect, the invention features a method of obtaining 3D surface data of an intra-oral cavity. A 3D measurement device having a measurement field of view is provided. A first scanning step includes moving the 3D measurement device within an intra-oral cavity so that the measurement field of view scans across a first surface of a dental arch. During the first scanning step, 3D data for the first surface are acquired by the 3D measurement device. The 3D data for the first surface define a backbone 3D data set for the dental arch. A second scanning step includes moving the 3D measurement device within the intra-oral cavity so that the measurement field of view scans across a second surface of the dental arch. The second surface at least partially overlaps the first surface. During the second scanning step, 3D data for the second surface are acquired by the 3D measurement device. The 3D data for the second surface are geometrically registered with the 3D data for the first surface to produce a registration result. Based on the registration result, the 3D data for the second surface are joined with the 3D data for the first surface.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and further advantages of this invention may be better understood by referring to the following description in conjunction with the accompanying drawings, in which like numerals indicate like structural elements and features in the various figures. For clarity, not every element may be labeled in every figure. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

FIG. 1 is a block diagram showing an example of a measurement system that can be used to obtain a 3D image of an object scene.

FIG. 2 illustrates a maneuverable wand that is part of a 3D measurement system used to obtain 3D measurement data for an intra-oral cavity.

FIG. 3 illustrates how an upper dental arch is measured using a handheld 3D measurement device such as the maneuverable wand of FIG. 2.

FIG. 4 a flowchart representation of an embodiment of a method of obtaining 3D surface data for a dental arch according to the invention.

FIG. 5 shows a measurement field of view at five different positions along an upper dental arch during 3D data acquisition for an occlusal scan according to the method of FIG. 4.

FIG. 6A and FIG. 6B are an occlusal view and a buccal view, respectively, of a wand and the corresponding location of the measurement field of view during a scan segment of a buccal surface.

DETAILED DESCRIPTION

The present teaching will now be described in more detail with reference to exemplary embodiments thereof as shown in the accompanying drawings. While the present teaching is described in conjunction with various embodiments and examples, it is not intended that the present teaching be limited to such embodiments. On the contrary, the present teaching encompasses various alternatives, modifications and equivalents, as will be appreciated by those of skill in the art. Those of ordinary skill in the art having access to the teaching herein will recognize additional implementations, modifications and embodiments, as well as other fields of use, which are within the scope of the present disclosure as described herein.

The methods of the present invention may include any of the described embodiments or combinations of the described embodiments in an operable manner. In brief overview, embodiments of the methods of the invention enable an accurate 3D measurement of one or more object surfaces. In various embodiments described below, the methods relate to the acquisition of 3D data during a 3D measurement procedure. The methods are described with respect to measurements of an oral cavity, such as a measurement made by a clinician in a dental application in which measured surfaces may include the enamel surface of teeth, the dentin substructure of teeth, gum tissue and various dental structures (e.g., posts, inserts and fillings). It will be appreciated that the methods can also be applied in medical applications and other applications in which 3D measurement data are acquired with 3D scanning devices under direct manipulation by an operator or otherwise maneuvered by a control system.

In the embodiments described below, 3D measurement systems use structured illumination patterns generated by interferometric fringe projection or other techniques. Imaging components acquire 2D images used to determine positional information of points on the surface of objects based on the structured illumination of the objects.

U.S. Pat. No. 5,870,191, incorporated herein by reference, describes a technique referred to as Accordion Fringe Interferometry (AFI) that can be used for high precision 3D measurements based on interferometric fringe projection. AFI-based 3D measurement systems typically employ two closely-spaced coherent optical sources to project the interferometric fringe pattern onto the surface of the object. Images of the fringe pattern are acquired for at least three spatial phases of the fringe pattern.

FIG. 1 illustrates an AFI-based 3D measurement system 10 used to obtain 3D images of one or more objects 22. Two coherent optical beams 14A and 14B generated by a fringe projector 18 are used to illuminate the surface of the object 22 with a pattern of interference fringes 26. An image of the fringe pattern at the object 22 is formed by an imaging system or lens 30 onto an imager that includes an array of photodetectors 34. For example, the detector array 34 can be a two-dimensional charge coupled device (CCD) imaging array. An output signal generated by the detector array 34 is provided to a processor 38. The output signal includes information on the intensity of the light received at each photodetector in the array 34. An optional polarizer 42 is oriented to coincide with the main polarization component of the scattered light. A control module 46 controls parameters of the two coherent optical beams 14 emitted from the fringe projector 18. The control module 46 includes a phase shift controller 50 to adjust the phase difference of the two beams 14 and a spatial frequency controller 54 to adjust the pitch, or separation, of the interference fringes 26 at the object 22.

The spatial frequency of the fringe pattern is determined by the separation of two virtual sources of coherent optical radiation in the fringe projector 18, the distance from the virtual sources to the object 22, and the wavelength of the radiation. The virtual sources are points from which optical radiation appears to originate although the actual sources of the optical radiation may be located elsewhere. The processor 38 and control module 46 communicate to coordinate the processing of signals from the photodetector array 34 with respect to changes in phase difference and spatial frequency, and the processor 38 determines 3D information for the object surface according to the fringe pattern images.

The processor 38 calculates the distance from the imaging system 30 and detector array 34 to the object surface for each pixel based on the intensity values for the pixel in the series of 2D images generated after successive phase shifts of the fringe patterns. Thus the processor creates a set of 3D coordinates that can be displayed as a point cloud or a surface map that represents the object surface. The processor 38 communicates with a memory module 58 for storage of 3D data generated during a measurement procedure. A user interface 62 includes an input device and a display to enable an operator such as a clinician to provide operator commands and to observe the acquired 3D information in a near real-time manner. For example, the operator can observe a display of the growth of a graphical representation of the point cloud or surface map as different regions of the surface of the object 22 are measured and additional 3D measurement data are acquired.

FIG. 2 illustrates a handheld 3D measurement device in the form of a maneuverable wand 66 that can be used to obtain 3D measurement data for an intra-oral cavity. The wand 66 includes a body section 70 that is coupled through a flexible cable 74 to a processor and other system components (not shown). The wand 66 generates a structured light pattern 78 that is projected from near the projection end 82 to illuminate a surface to be measured. For example, the structured light pattern 78 can be an interferometric fringe pattern based on the principles of an AFI measurement system as described above for FIG. 1. The wand 66 can be used to obtain 3D data for a portion of a dental arch. The wand 66 is maneuvered within the intra-oral cavity by a clinician so that 3D data are obtained for all surfaces that can be illuminated by the structured light pattern 78.

FIG. 3 illustrates an intra-oral application in which an upper dental arch is measured using a handheld 3D measurement device such as the wand 66 of FIG. 2. Reference is also made to FIG. 4 which presents a flowchart representation of an embodiment of a method 100 of obtaining 3D surface data for a dental arch. The measurement results in a complete 3D data set that accurately represents the full arch of a patient, i.e., from the back molar on one side of the arch to the back molar on the opposite side of the arch. During data processing, stitching errors and motion-induced errors can degrade the measurement results. For a 3D measurement device that includes a 2D imager with a small measurement field of view (FOV) (e.g., 13 mm×10 mm) relative to the full arch, hundreds of 3D data sets may be stitched together to obtain the complete 3D data set for the arch.

According to the method 100, numerous overlapping 3D data sets are stitched together in a common coordinate reference. The 3D data are obtained in a preferred manner or sequence so that the “final” 3D data set resulting from all the 3D data more accurately represents the dental arch. In particular, a backbone 3D data set is first generated and additional sequences of 3D data are subsequently joined to the backbone 3D data set. Individual scan segments are used to acquire subsets of 3D data for the final point cloud. Limited motion of the wand during the 3D data acquisition for each scan segment results in reduced measurement error and increased measurement accuracy.

A clinician performs the 3D measurement method 100 by positioning (step 105) the wand so that the structured light pattern illuminates a portion of an occlusal surface of the dental arch at a starting location, for example, at one end of the dental arch. 3D data are acquired for the illuminated portion of the occlusal surface. In this example, data acquisition starts by acquiring data from within a measurement field of view 86A at the patient's left back molar 90 of the upper dental arch as shown in FIG. 3. The wand is then moved (step 110) from the patient's left back molar 90 along the arch to the right back molar while maintaining a substantially occlusal view. FIG. 5 shows the measurement field of view 86A to 86E for several positions (five positions A to E) along the arch. The full occlusal scan does not require significant rotation of the wand during this portion of the 3D measurement procedure. Rotation and focus induced errors are therefore reduced in comparison to scans obtained by manipulating the wand for other views of the arch. In some embodiments, the motion of the wand is substantially in a direction parallel to the stripes or fringes of the structured light pattern. The primary motion of the wand during the occlusal scan is restricted substantially to a single plane. Advantageously, the occlusal view includes features having substantially higher spatial frequency content than other views. That is, the occlusal view more readily shows rapidly varying structure (e.g., gaps between teeth) that improve the accuracy for stitching of 3D data relative to other scan views (i.e., buccal and lingual). Thus the 3D data corresponding to the occlusal scan defines a backbone that can be used for attachment by other 3D data sets obtained during other directional scan views as necessary to obtain a complete set of 3D data for the arch.

To continue the measurement procedure, the clinician positions (step 115) the wand so that the structured light pattern illuminates a portion of the occlusal surface, for example, near or at one end of the arch, and 3D data are acquired that overlap a portion of the backbone 3D data set. Preferably, the 3D measurement system provides an affirmative visual or audible indication to the clinician when the new 3D data for the real-time position of the structured light pattern “locks on” to the display of a point cloud for the backbone 3D data set. The newly-acquired 3D data are then registered or joined (step 120) to the backbone 3D data and serve as the start of a buccal scan segment for the arch. The wand is then rotated about its primary axis and moved (step 125) so that a portion of the buccal surface of the arch is illuminated with the structured light pattern and 3D data are acquired. The wand is then maneuvered (step 130) by the clinician so that the structured light pattern moves along a segment of the buccal surface. For example, the wand may be moved so that the structured light pattern is scanned in time from the patient's back left molars to just beyond the midpoint of the buccal surface. FIG. 6A and FIG. 6B show an occlusal view and a buccal view, respectively, of the position of the projection end 82 of the wand and the corresponding location of the measurement field of view (and structured light pattern) part way through this scan segment of the buccal surface. Optionally, the clinician can rotate (step 135) the wand so that the structured light pattern illuminates the midpoint of the occlusal surface. In this manner, the structured light pattern is used to acquire 3D data in an occlusal view that overlay data in the backbone 3D data set to more accurately “register” to the existing 3D data in the common coordinate reference system.

A complementary buccal scan segment can now be performed. The clinician positions (step 140) the wand so that the structured light pattern illuminates a portion of the occlusal surface at the opposite end of the arch and 3D data are acquired that overlap a portion of the backbone 3D data set. The 3D data at this location are joined (step 145) to the 3D backbone data set and serve as the start of a complementary buccal scan segment for the arch. The wand is then rotated about its primary axis (step 150) so that a portion of remainder of the buccal surface of the arch is illuminated with the structured light pattern and 3D data are acquired. Subsequently, the wand is maneuvered (step 155) by the clinician so that the structured light pattern moves along the remainder of the buccal surface segment. For example, the wand may be moved so that the structured light pattern is moved in time from the patient's back right molars to just beyond the midpoint of the buccal surface. Optionally, the clinician can rotate (step 160) the wand so that the structured light pattern illuminates the midpoint region of the occlusal surface and 3D data are acquired in an occlusal view that overlay the data in the backbone 3D data set. Thus the 3D data for this buccal segment are accurately registered in the common reference system of the backbone 3D data set.

To complete the 3D measurement of the arch, the clinician obtains 3D data for the lingual surface of the dental arch in a manner similar to that used for obtaining 3D data for the buccal surface. More specifically, steps 125 to steps 160 are performed by replacing all references to the buccal surface with references to the lingual surface. In total, five scan segments are performed to obtain a full set of 3D data for the final 3D data set for the dental arch.

In effect, the steps of joining 3D data to the backbone 3D data set allows sequences of individual 3D images to be attached by referring to a subset of the chronologically ordered 3D images in the backbone 3D data set. This joining technique “primes the stitcher” so that the subsequent scan is properly registered to the backbone 3D data set and accurately shares the same global coordinate system.

In an alternative embodiment, the order of scan segments can differ. For example, acquisition of the 3D data for the two lingual segments can precede acquisition of the 3D data for the buccal segments.

In other embodiments, the clinician can use a greater number of buccal and lingual scan segments and the extent of each segment can be smaller. In such embodiments, the measurement system displays various portions of the backbone 3D data set to allow joining the backbone 3D data set at other locations.

In the embodiments described above for the method 100, the structured light pattern and measurement field of view used for 3D data acquisition are moved along various surfaces of a dental arch and repositioned by manipulating the position and rotation of a wand. The method can be adapted for other types of 3D measurement systems. For example, the method can be performed using a measurement field of view of a wand or maneuverable 3D measurement device that can be translated, rotated and positioned in a similar manner to the structured light pattern such that 3D data initially generated during the procedure can be used to generate a backbone 3D data set and subsequent 3D data can be joined to the backbone 3D data set to obtain a high accuracy 3D data representation of an object scene. Furthermore, the method preferably obtains 3D measurement data first for a directional view that substantially requires only two dimensional translation of the measurement field of view and in which high spatial frequency content is observable to create the backbone 3D data set and subsequent directional views are used to generate additional 3D data that can be attached to the backbone 3D data set.

While the invention has been shown and described with reference to specific embodiments, it should be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the invention as recited in the accompanying claims. 

What is claimed is:
 1. A method of obtaining three-dimensional (3D) surface data of an intra-oral cavity, the method comprising: providing a 3D measurement device having a measurement field of view; moving the 3D measurement device during a first scan segment within the intra-oral cavity so that the measurement field of view scans across a first surface of a dental arch while acquiring, by the 3D measurement device, 3D data for the first surface, the first surface being one of an occlusal surface, a buccal surface and a lingual surface of the dental arch, the 3D data for the first surface defining a backbone 3D data set for the dental arch; moving the 3D measurement device after completion of the first scan segment to a position wherein the measurement field of view at least partially overlaps the first surface; initiating acquisition of 3D data at the position at a beginning of a second scan segment; moving the 3D measurement device during the second scan segment within the intra-oral cavity so that the measurement field of view scans across a second surface of the dental arch while acquiring, by the 3D measurement device, 3D data for the second surface, the second surface being a different one of the occlusal surface, the buccal surface and the lingual surface of the dental arch; geometrically registering the 3D data for the second surface with the 3D data for the first surface during the acquisition of 3D data for the second surface to produce a first registration result; and based on the first registration result, joining the 3D data for the second surface with the 3D data for the first surface during the acquisition of 3D data for the second surface.
 2. A method according to claim 1, further comprising: moving the 3D measurement device within the intra-oral cavity so that the measurement field of view scans across a third surface of the dental arch while acquiring, by the 3D measurement device, 3D data for the third surface, the third surface at least partially overlapping the first surface and being a different one of the occlusal surface, buccal surface and lingual surface of the dental arch with respect to the first and second surfaces; geometrically registering the 3D data for the third surface with the 3D data for the first surface during the acquisition of 3D data for the third surface to produce a second registration result; and based on the second registration result, joining the 3D data for the third surface with the 3D data for the first surface during the acquisition of 3D data for the third surface.
 3. A method according to claim 2, wherein the first surface is the occlusal surface of the dental arch, the second surface is the buccal surface of the dental arch, and the third surface is the lingual surface of the dental arch.
 4. A method according to claim 1, wherein the first surface is the occlusal surface of the dental arch.
 5. A method according to claim 4, wherein the second surface is the buccal or the lingual surface of the dental arch.
 6. A method according to claim 1, wherein the 3D measurement device is an accordion fringe interferometer measurement device. 